Flat panel displays are widely available to consumers. For example, liquid crystal displays (LCDs) are standard for computer use and are available for home theater televisions. The LCD panel has a liquid crystal layer sandwiched between two transparent substrates and is illuminated by a backlight module. FIG. 1A is a cross-sectional view of a conventional backlight module. The conventional backlight module 10 includes a reflecting sheet 2, a light-guide plate 1, a lamp 6, a lateral housing 3, and back 4 and top 5 housings. The reflecting sheet 2 is attached to a bottom surface of a light-guide plate 1 and a lamp 6 is disposed near a lateral surface of the light-guide plate 1. The lateral housing 3 tightly clamps the reflecting sheet 2 and the light-guide plate 1 together. This assembly is placed on the back housing 4; the top housing 5 is connected to the back housing 4. To embed the reflecting sheet 2 and the light-guide plate 1 into the lateral housing 3, the lateral housing 3 is temporarily deformed. This method of assembly wastes time and labor costs, and it is inefficient.
If, however, the lateral housing is not used, the reflecting sheet and the light-guide plate may not be completely sealed together. For example, the light-guide plate may form a positive buckle (FIG. 1B at 20) or a negative buckle (FIG. 1C at 70) due to one or more of residual stress, temperature change, and humidity change. With the positive buckle, the two edges of the light-guide plate 20 are higher than the middle portion, and with the negative buckle, the middle portion of the light-guide plate 70 is higher than its two edges. Because negatively-buckled light-guide plate 70 arches so that the middle portion contacts a display panel 60, the display quality may be influenced, and the light-guide plate 70 and a reflecting sheet 80 (which is disposed on a back housing 90) cannot be sealed together. To prevent the light-guide plate from contacting the panel, the buckle has to be limited to the positive buckle.
Referring back to the positive buckle shown in FIG. 1B, and taking a 17″ panel as an example, the distance d2 from the highest point to the lowest point of the light-guide plate 20 cannot be greater than 0.5 millimeters (mm). In this example, the back housing 40 may form a positive buckle or a negative buckle (the middle portion of the back housing 40 is higher than the two edges of the back housing 40). The conventional reflecting sheet 30 is a thin sheet that is attached to a back housing 40; thus, if the back housing 90 buckles, the reflecting sheet 30 buckles as well. Moreover, if the disposed reflecting sheet 30 has the negative buckle the distance d1 from the highest point to the lowest point of the reflecting sheet 30 should be smaller than or equal to 0.5 mm. Nevertheless, the maximum distance from the highest point of the positively buckled light-guide plate 20 to the lowest point of the negatively buckled reflecting sheet 30 may reach 1 mm. This distance is sufficient to enable light L outputted by the lamp 50 to directly irradiate on the reflecting sheet 30 and to be reflected to form a bright zone on a display panel surface, thereby influencing the external appearance of the product.
Thus, there is a need for a backlight module that reduces the gap between the reflecting sheet and the light-guide plate without using a clamping lateral housing.